1. Field of the Invention
The present invention relates to a power supply device that may be applied to a heating device of electromagnetic inducting heating type, or the like, and also relates to a fixing device that uses the heating device for fixing images in an image forming apparatus.
2. Description of Related Art
Image heating devices (or fixing devices) have been conventionally installed in image forming apparatuses. In a known example that will be discussed below, a fixing device is installed in an image forming apparatus, such as a copying machine or a printer, for the purpose of heating and fixing a toner image on a recording medium.
In image forming apparatuses wherein an unfixed image (toner image) of desired image information is formed by a transfer method or direct method on a recording medium or material (such as a transfer sheet, electrofax sheet, electrostatic recording paper, OHP sheet, printing paper, or format paper), by a suitable image forming process, such as an electrophotographic process, electrostatic recording process, or a magnetic recording process, heat roller type devices were widely used as fixing devices for heating and fixing the unfixed image (toner image) on the surface of the recording medium, to provide a permanent fixed image. More recently, belt heating type fixing devices have been used in practice in view of quick start and energy conservation. Electromagnetic induction heating type devices are also proposed. In the following description, various types of fixing devices used in image forming apparatuses will be explained.
(a) Fixing Device of Heat Roller Type
A fixing device of heat roller type is basically constituted by a pair of rollers, i.e., a fixing roller (heating roller) and a pressure roller that are in pressed contact with each other. With the pair of rollers being rotated, a recording medium that carries an unfixed toner image thereon is nipped and transported by a fixing nip portion at which the rollers are in pressed contact with each other, so that the unfixed toner image is fixed on the surface of the recording medium, utilizing the heat of the fixing roller and the pressing force of the fixing nip portion.
The fixing roller generally has a hollow metal roller made of aluminum, as a base (core), and a halogen lamp as a heat source which is inserted into and placed in the aluminum roller. In operation, the halogen lamp generates heat for heating the fixing roller, and electric power to be supplied to the halogen lamp is controlled so that the outer peripheral surface of the roller is maintained at a certain fixing temperature.
In particular, a fixing device of an image forming apparatus adapted for forming full-color images is required to have an ability to sufficiently heat and fuse a maximum of four layers of toner images to mix different colors. To this end, the core of the fixing roller is made of a material having a large thermal capacity, and a rubber elastic layer is provided on the outer periphery of the core, for surrounding and uniformly fusing a toner image, so that the toner image is heated via the rubber elastic layer. A heat source is placed in a pressure roller as well as in a fixing roller (heating roller), and the pressure roller is heated with its temperature being suitably controlled.
The fixing device of heat roller type as described above, however, has a disadvantage as follows: when the power supply of the image forming apparatus is turned on, and the halogen lamp as the heat source of the fixing device starts being energized at the same time, it requires a considerably long time (waiting time) for the fixing roller having a large thermal capacity and other components to be heated from a totally cold state to a certain temperature that enables fixing. Thus, the fixing device of heat roller type has a poor quick start capability. Also, there is a need to energize the halogen lamp to keep the fixing roller in a certain temperature controlled state while the image forming apparatus is being in the standby state (i.e., while images are not being generated), so that the image forming operations can be carried out at any time. Thus, the fixing device of this type suffers from a large amount of power consumption.
In the case of the fixing device of the above-described full-color image forming apparatus, which uses a fixing roller having a large thermal capacity, an increase in the temperature at the surface of the fixing roller is delayed relative to the desired timing of temperature control, thus causing poor fixing, gloss variations, offset and other problems.
(b) Fixing Device of Film Heating Type
Fixing devices of film heating type have been proposed in, for example, Laid-open Japanese Patent Publications (Kokai) Nos. 63-313182, 2-157878, 4-44075 and 4-204980.
The fixing device of the above type is constructed such that a heat resistant film (fixing film) is sandwiched between a ceramic heater as a heating body and a pressure roller as a pressure member, to thus form a nip portion. A recording medium that carries an unfixed toner image thereon is pinched or nipped between the film and the pressure roller in the nip portion, and fed along with the film. In this manner, heat generated by the ceramic heater is transferred to the recording medium via the film in the nip portion, and the unfixed toner image is thermally fixed onto the surface of the recording medium, using the heat and the pressing force of the nip portion.
The fixing device of film heating type may be constructed as that of on demand type, by using a ceramic heater and a film both having a low thermal capacity. In operation, the ceramic heater as a heat source is energized only during execution of the image forming process of the image forming apparatus, so as to heat the fixing device to a certain fixing temperature. Thus, the fixing device of this type is advantageous in a relatively short waiting time (improved quick start ability) as measured from the turn-on of the power supply of the image forming apparatus until the apparatus is ready to execute the image forming process, and also advantageous in significantly reduced power consumption (power saving) during standby. The fixing device, however, has a difficulty in terms of the thermal capacity when it is used in full-color image forming apparatuses or high-speed machines.
(c) Fixing Device of Electromagnetic Inducting Heating Type
Laid-open Publication No. 51-109739 of Japanese Utility Model Application discloses an induction heating type fixing device in which magnetic flux is used for inducing current in a fixing roller, thereby to generate heat in the form of Joule heat. This type of fixing device makes it possible to directly heat the fixing roller by utilizing induction current, thus achieving a highly efficient fixing process, as compared with the fixing device of heat roller type which uses a halogen lamp as a heat source.
In the fixing device of the induction heating type, however, the energy of alternating magnetic flux generated by an exciting coil serving as magnetic field generating means is used for increasing the temperature of the whole fixing roller, resulting in an increased heat radiation loss, and a reduced ratio of the fixing energy to the energy put into the system, or reduced efficiency.
In view of the above problem, a highly efficient fixing device has been proposed in which the exciting coil is located close to the fixing roller as a heat generating body so as to provide the energy for fixing with a high efficiency, or the distribution of alternating magnetic flux of the exciting coil is concentrated at around the fixing nip portion.
Referring to FIG. 24, one example of fixing device of electromagnetic induction heating type will be briefly described wherein the distribution of alternating magnetic flux of the exciting coil is concentrated at around the fixing nip portion so as to improve the efficiency.
In FIG. 24, reference numeral 10 denotes a cylindrical fixing film (fixing belt) which serves as a rotator capable of generating heat due to electromagnetic induction, and which has an electromagnetic induction heat generating layer (conductive layer, magnetic layer, resistive layer). The cylindrical fixing film 10 is loosely disposed around a gutter-like film guide (belt guide) member having a substantially semicircular shape in transverse cross section. Magnetic field generating means 15 that is disposed inside the film guide member 16 is comprised of an exciting coil 18 and an E-shaped magnetic core (core member) 17. An elastic pressure roller 30 is in pressed contact with the lower surface of the film guide member 16 under a certain pressure, with the fixing film 10 sandwiched between the pressure roller 30 and the film guide member 16, to thus form a fixing nip portion N having a certain width. The magnetic core 17 of the magnetic field generating means 15 is located in alignment with the fixing nip portion N.
The pressure roller 30 is driven by driving means M to rotate in the counterclockwise direction as indicated by an arrow in FIG. 24. A force is applied to rotate the fixing film 10 due to the frictional force that arises between the pressure roller 30 and the outer surface of the fixing film 10 as a result of the rotary movement of the pressure roller 30. Thus, the fixing film 10 is caused to rotate around the belt guide member 16 in the clockwise direction as indicated by an arrow in FIG. 24, at a peripheral velocity substantially equal to the peripheral velocity of rotation of the pressure roller 30, while the inner surface of the fixing film 10 is sliding along the adhering lower surface of the film guide member 16 in the fixing nip portion N.
The film guide member 16 functions to apply a pressure to the fixing nip portion N, support the exciting coil 18 and magnetic core 17 as magnetic field generating means 15, support the fixing film 10, and provide sufficient transport stability during rotation of the fixing film 10. The film guide member 16 is an insulating member that inhibits passage of magnetic flux therethrough, and is formed of a material that is able to stand a heavy load.
The exciting coil 18 generates alternating magnetic flux when alternating current is supplied from an exciting circuit (not shown) to the coil 18. The E-shaped magnetic core 17 causes the alternating magnetic flux to be concentrated in the fixing nip portion N, and the alternating magnetic flux causes eddy current to flow through the electromagnetic induction heat generating layer of the fixing film 10 in the fixing nip portion N. With the eddy current thus produced, the electromagnetic induction heat generating layer generates Joule heat due to the specific resistance of the heat generating layer.
The heat generation of the fixing film 10 due to electromagnetic induction occurs and is concentrated at the fixing nip portion N in which the alternating magnetic flux is highly distributed or concentrated, so that the fixing nip portion N is heated with a high efficiency. To control the temperature of the fixing nip portion N, current to be supplied to the exciting coil 18 is controlled by a temperature control system including a temperature detecting device (not shown) so that the temperature of the fixing nip portion N is maintained at a certain temperature.
As described above, the pressure roller 30 is driven or rotated, and the cylindrical fixing film 10 is rotated around the film guide member 16, while electric power is supplied from the exciting circuit to the exciting coil 18 so that the fixing nip portion N is heated to a certain target temperature by heat generation of the fixing film 10 due to electromagnetic induction, and maintained at the target temperature. In this state, a recording medium P on which an unfixed toner image xe2x80x9ctxe2x80x9d is formed and which is fed from an image forming portion (not shown) is introduced into the fixing nip portion N to be nipped between the fixing film 10 and the pressure roller 30, with its image carrying surface facing upward, i.e., toward the surface of the fixing film. The image carrying surface of the recording medium adheres to the outer surface of the fixing film 10 in the fixing nip portion N, and the recording medium P and the fixing film 10 are fed through the fixing nip portion N while being nipped between the film guide member 16 and the pressure roller 30.
While the recording medium P is being fed along with the fixing film 10 through the fixing nip portion N, the recording medium P is heated due to electromagnetic induction of the fixing film 10, and the unfixed toner image xe2x80x9ctxe2x80x9d on the recording medium P is heated and fixed. After passing through the fixing nip portion N, the recording medium P is separated from the outer surface of the rotating fixing film 10, and continues to be transported.
Inverter circuits used in power supplies for electromagnetic induction heating constructed as above are roughly classified into those of current resonance type and those of voltage resonance type. The resonance method is employed for the reason as follows: where a relatively large electric power is to be handled, an oscillating state of voltage or current that occurs upon switching of a switching device for conversion is positively or deliberately produced so as to reduce loss of the switching device, and the switching device is turned on/off when the voltage or current or both is at the lowest level. This method is called xe2x80x9csoft switchingxe2x80x9d, which is the most effective method when handling large electric power, and various methods have been proposed.
FIG. 25 shows an inverter circuit of voltage resonance type as a known example. The inverter circuit includes a switching device 202, a resonance coil (exciting coil) 203, and a resonating capacitor 205. In the operation of the known voltage resonance inverter, when the switching device 202 is turned off after the device 202 is turned on and electric power is stored in the resonance coil 203, the voltage starts oscillating while drawing an arc of resonance, with a frequency that is determined by the constants of the resonance coil 203 and the resonating capacitor 205.
In FIG. 26, 208 indicates the gate voltage waveform of the switching device 202, and 210 indicates the current waveform of the switching device 202 and a diode 207, while 211 indicates the voltage waveform of the switching device 202.
FIG. 27 shows the operating waveform when the ON duration of the gate switching signal is shortened so as to restrict output power, and a power converting operation is performed. The voltage waveform 211 of the switching device 202 when the output power is restricted shows a sine wave that resonates about the power supply voltage (whose level is indicated by a broken line) as a reference, with the power supply being connected to the terminal of the resonance coil 203. The oscillation amplitude of the voltage depends upon the exciting electric power stored in the resonance coil (exciting coil) 203, namely, the ON duration of the switching signal of the switching device 202. The oscillation amplitude is small during a power-saving operation, and the voltage is not sufficiently lowered from the level of the power supply voltage, and fails to cross zero.
Namely, the switching device 202 performs switching of a considerably low-impedance load of the resonating capacitor 205 via the power supply line, and excessively large current is caused to flow through the switching device 202 upon turn-on. With the voltage resonance power supply, the range in which the switching device 202 does not break down can be only restricted to about ⅓ of the maximum output due to the excessively large current, which makes it difficult to design the circuit.
The following problems, however, have occurred in the prior art as described above. In general, the required width of the electric power control region used by the fixing heating device installed in the image forming apparatus is 1100 W to 150 W. While the operating methods of induction heating power supplies proposed by the present invention are roughly classified into a current resonance method and a voltage resonance method, and the voltage resonance method that can be realized with a simple configuration is widely employed.
In the electric power control according to the conventional voltage resonance method, the power can be restricted to about one-third of the maximum output, namely, down to 350 W in the above example. If the power is to be further restricted, the fixing device is no longer held in the voltage resonance state, and large current flows through the switching device and breaks down the device.
If fixing control is performed with the power supply as described above in the image forming apparatus, the power of 350 W is excessively large while the fixing device is in the temperature saturated state during continuous printing, and the circuit performs intermittent operations. Under the control performed with such intermittent operations, the temperature tends to be unstable, and the voltage resonance circuit, which is formed of a parallel circuit of a resonating capacitor and a coil, has excessively large current flowing therethrough upon start, which current gives stress to the switching device.
As an electric circuit for heating control according to the electromagnetic induction heating method, a voltage resonance circuit is known which is comprised of a switching device that is connected in series with an exciting coil, a resonating capacitor that is connected in parallel with the switching device, and a filter capacitor provided in the later stage of an AC line voltage rectifier. With the voltage resonance circuit thus constructed, the switching device is turned on/off when the voltage applied to the switching device becomes equal to xe2x80x9c0xe2x80x9d, utilizing flyback voltage that is generated by the parallel resonance circuit of the coil and capacitor.
In the voltage resonance circuit as described above, however, where the output power is small, namely, where the energy per pulse is small, the magnitude of the flyback voltage is not sufficient, and soft switching operations cannot always be expected.
It is therefore an object of the present invention to provide a power supply device and a voltage resonance method which are free from the drawbacks as described above.
Another object of the present invention is to provide a power supply device and a voltage resonance method which are free from a reduced switching loss.
A further object of the present invention is to provide a power supply device and a voltage resonance method which are free from the situation in which soft switching operations for generating an alternating magnetic field cannot be executed.
A still further object of the present invention is to provide a fixing device that employs the power supply device and voltage resonance method as described above.
To attain the above objects, according to a first aspect of the present invention, there is provided a power supply device comprising a first switching device, a first charging device connected in series with the first switching device, a second switching device connected in series with the first charging device, a magnetic field generating device that is connected between a power supply line and a node between the first charging device and the second switching device, a second charging device connected in parallel with the second switching device, a first rectifying device connected in parallel with the first switching device, a second rectifying device connected in parallel with the second switching device, and a switching control circuit that controls driving of the first and second switching devices.
To attain the above objects, according to a second aspect of the present invention, there is provided a power supply device, comprising a first switching device, a first charging device connected in series with the first switching device, a second switching device connected in series with the first charging device, a magnetic field generating device that is connected between a node of the first charging device and the second switching device, and a power supply line, a second charging device connected in parallel with the magnetic field generating device, a first rectifying device connected in parallel with the first switching device, a second rectifying device connected in parallel with the second switching device, and a switching control circuit that controls driving of the first and second switching devices.
In the power supply device according to the first and second aspects, the switching control circuit alternately turns on the first switching device and the second switching device.
More specifically, the switching control circuit turns off the first switching device before turning on the second switching device.
The magnetic field generating device and the first charging device constitute a first resonance circuit, and the magnetic field generating device and the second charging device constitute a second resonance circuit.
Preferably, the second charging device has a capacity that is sufficiently smaller than that of the first charging device.
In a typical application of the present invention, the power supply device is provided in a heat generating device that generates heat utilizing electromagnetic induction of the magnetic field generating device.
The power supply device can be advantageously used in an image recording apparatus that employs an electrophotographic recording method, in which the heat generating device is a fixing device of the image recording apparatus.
To attain the above objects, according to a third aspect of the present invention, there is provided a voltage resonance method for use in a power supply device comprising a first switching device, a first charging device connected in series with the first switching device, a second switching device connected in series with the first charging device, a magnetic field generating device that is connected between a power supply line and a node between the first charging device and the second switching device, a second charging device connected in parallel with the second switching device, a first rectifying device connected in parallel with the first switching device, and a second rectifying device connected in parallel with the second switching device, the voltage resonance method comprising the steps of turning on the first switching device, turning off the first switching device before turning on the first switching device, and turning on the second switching device after turning off the first switching device.
To attain the above objects, according to a fourth aspect of the present invention, there is provided a voltage resonance method for use in a power supply device comprising a first switching device, a first charging device connected in series with the first switching device, a second switching device connected in series with the first charging device, a magnetic field generating device that is connected between a power supply line and a node between the first charging device and the second switching device, a second charging device connected in parallel with the magnetic field generating device, a first rectifying device connected in parallel with the first switching device, and a second rectifying device connected in parallel with the second switching device, the voltage resonance method comprising the steps of turning on the first switching device, turning off the first switching device before turning on the second switching device, and turning on the second switching device after turning off the first switching device.
In a typical application of the present invention, the voltage resonance method according to the third and fourth aspects may be applied to a power supply device that is provided in a heat generating device that generates heat utilizing electromagnetic induction of the magnetic field generating device.
The voltage resonance method according to the present invention can be advantageously applied to an image recording apparatus that employs an electrophotographic recording method, in which the heat generating device is a fixing device of the image recording apparatus.
To attain the above objects, according to a fifth aspect of the present invention, there is provided a power supply device comprising a magnetic field generating device, a first switching device connected in series with the magnetic field generating device, a first charging device connected in series with the first switching device, a second switching device connected in parallel with a line on which the first switching device and the first charging device are connected in series, a second charging device connected in parallel with the second switching device, a first rectifying device connected in parallel with the first switching device, a second rectifying device connected in parallel with the second switching device, and a switching control circuit that controls driving of the first and second switching devices.
Preferably, the power supply device according to the fifth aspect further comprises a power detecting circuit that detects electric power supplied to the magnetic field generating device, and wherein the switching control circuit turns on the first switching device before turning on the second switching device, depending upon the electric power detected by the power detecting circuit.
More preferably, the power detecting circuit detects current that flows through the first switching device.
Preferably, the power supply device according to the fifth aspect further comprises a voltage detecting circuit that detects flyback voltage that appears in the magnetic field generating device, and wherein the switching control circuit turns on the first switching device before turning on the second switching device when the flyback voltage detected by the voltage detecting circuit is equal to or smaller than a predetermined value.
Further preferably, the power supply device according to the fifth aspect further comprises a voltage detecting circuit that detects a voltage of a power supply line, and a temperature detecting circuit that detects a temperature of a heat generating body that generates heat utilizing a magnetic field generated by the magnetic field generating device, and wherein the switching control circuit turns on the first switching device before turning on the second switching device when the voltage detected by the voltage detecting circuit is equal to or larger than a predetermined value, and an ON duration of the second switching device is reduced to be smaller than a predetermined value while the heat generating body is maintained at a predetermined temperature based on an output of the temperature detecting circuit.
Alternatively, the power supply device according to the fifth aspect further comprises a current detecting circuit that detects current that flows through the first rectifying device, and a temperature detecting circuit that detects a temperature of a heat generating body that generates heat utilizing a magnetic field generated by the magnetic field generating device, and wherein the switching control circuit turns on the first switching device before turning on the second switching device when the current detected by the current detecting circuit is equal to or larger than a predetermined value, and an ON duration of the second switching device is reduced to be smaller than a predetermined value while the heat generating body is maintained at a predetermined temperature based on an output of the temperature detecting circuit.
Other objects of the invention will become apparent from the following description based on the accompanying drawings, and the appended claims.